Convert documentation from RST to Markdown using MyST-Parser

Replace all 25 .rst files under docs/ with MyST Markdown equivalents,
add myst-parser as a dev dependency, and configure Sphinx to support
both .rst and .md source files for backward compatibility with older
tagged branches. Closes #652.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: laughingman7743

docs/api.md

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+(api)=
+
+# API reference
+
+This section provides comprehensive API documentation for all PyAthena classes and functions, organized by functionality.
+
+```{toctree}
+:maxdepth: 2
+:caption: API Documentation:
+
+api/connection
+api/converters
+api/utilities
+api/errors
+api/filesystem
+api/models
+api/sqlalchemy
+api/pandas
+api/arrow
+api/polars
+api/s3fs
+api/spark
+```
+
+## Quick reference
+
+### Core functionality
+
+- {ref}`api_connection` - Connection management and basic cursors
+- {ref}`api_converters` - Data type conversion and parameter formatting
+- {ref}`api_utilities` - Utility functions and base classes
+- {ref}`api_errors` - Exception handling and error classes
+
+### Infrastructure
+
+- {ref}`api_filesystem` - S3 filesystem integration and object management
+- {ref}`api_models` - Athena query execution and metadata models
+
+### Specialized integrations
+
+- {ref}`api_sqlalchemy` - SQLAlchemy dialect implementations
+- {ref}`api_pandas` - pandas DataFrame integration
+- {ref}`api_arrow` - Apache Arrow columnar data integration
+- {ref}`api_polars` - Polars DataFrame integration (no pyarrow required)
+- {ref}`api_s3fs` - Lightweight S3FS-based cursor (no pandas/pyarrow required)
+- {ref}`api_spark` - Apache Spark integration for big data processing

docs/api.rst

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+(api_arrow)=
+
+# Apache Arrow Integration
+
+This section covers Apache Arrow-specific cursors, result sets, and data converters.
+
+## Arrow Cursors
+
+```{autoclass} pyathena.arrow.cursor.ArrowCursor
+:members:
+:inherited-members:
+```
+
+```{autoclass} pyathena.arrow.async_cursor.AsyncArrowCursor
+:members:
+:inherited-members:
+```
+
+## Arrow Result Set
+
+```{autoclass} pyathena.arrow.result_set.AthenaArrowResultSet
+:members:
+:inherited-members:
+```
+
+## Arrow Data Converters
+
+```{autoclass} pyathena.arrow.converter.DefaultArrowTypeConverter
+:members:
+```
+
+```{autoclass} pyathena.arrow.converter.DefaultArrowUnloadTypeConverter
+:members:
+```
+
+## Arrow Utilities
+
+```{autofunction} pyathena.arrow.util.to_column_info
+```
+
+```{autofunction} pyathena.arrow.util.get_athena_type
+```

docs/api/arrow.md

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+(api_connection)=
+
+# Connection and Cursors
+
+This section covers the core connection functionality and basic cursor operations.
+
+## Connection
+
+```{automodule} pyathena
+:members: connect
+```
+
+```{autoclass} pyathena.connection.Connection
+:members:
+:inherited-members:
+```
+
+## Standard Cursors
+
+```{autoclass} pyathena.cursor.Cursor
+:members:
+:inherited-members:
+```
+
+```{autoclass} pyathena.cursor.DictCursor
+:members:
+:inherited-members:
+```
+
+## Asynchronous Cursors
+
+```{autoclass} pyathena.async_cursor.AsyncCursor
+:members:
+:inherited-members:
+```
+
+```{autoclass} pyathena.async_cursor.AsyncDictCursor
+:members:
+:inherited-members:
+```
+
+## Result Sets
+
+```{autoclass} pyathena.result_set.AthenaResultSet
+:members:
+:inherited-members:
+```
+
+```{autoclass} pyathena.result_set.AthenaDictResultSet
+:members:
+:inherited-members:
+```
+
+```{autoclass} pyathena.result_set.WithResultSet
+:members:
+```

docs/api/arrow.rst

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+(api_converters)=
+
+# Data Conversion
+
+This section covers data type converters and parameter formatters.
+
+## Type Converters
+
+```{autoclass} pyathena.converter.Converter
+:members:
+```
+
+```{autoclass} pyathena.converter.DefaultTypeConverter
+:members:
+```
+
+## Parameter Formatters
+
+```{autoclass} pyathena.formatter.Formatter
+:members:
+```
+
+```{autoclass} pyathena.formatter.DefaultParameterFormatter
+:members:
+```